Litle account updater support

 - Adds account updater XML to the commerce_litle_test module.
 - Some other improvements to the commerce_litle_test module.
 - Adds a hook to alter the end date when creating a sustainer series.
 - Adds a setting to Litle configuration to extend new sustainers by one month.
 - Adds functionality to the Litle module to update a sustainer series when Litle sends account updater information.
 - Refactored the billing update code so that Account Updater can use it.

I ended up writing this set of tests back when we were planning on modifying the expire callback to the extend the series. We switched strategies but I had already written this.